Communication systems that geographically reuse communication resources are known in the art. These systems allocate a predetermined set of communication resources in one geographic area and reuse the same set of communication resources in one or more geographic areas. This reuse technique improves communication capacity by minimizing the number of communication resources necessary to provide communication service in a large geographic area comprised of several smaller geographic areas.
Two of the most common communication systems which geographically reuse communication resources are cellular and trunked mobile communication systems. In both communication systems, allocation of a communication resource begins when a communication unit requests communication service. Based on resource availability and signal usability, a resource controller assigns the communication resource, such as a frequency channel or a time slot, to the communication unit. A communication, such as a conversation or a facsimile transmission, occurs on the communication resource between the communication unit and another communication unit or between the communication unit and a subscriber to a public service telephone network. The communication continues until completion or an interruption in service occurs. Upon conclusion of the communication, the resource controller retrieves the communication resource; thus making the communication resource available for another communication.
An important parameter in identifying an acceptable communication resource is signal usability. In a wireless communication system, communication resources are typically radio frequency (RF) channels which occupy predetermined bandwidths. When information signals are transmitted on the RF channels (communication resources), undesired channel effects, such as interference, noise, and distortion of the transmitter and receiver, alter the information signals during transmission and reception. Therefore, the information signals received by a receiver in a communication unit, or a base station, are corrupted by the interfering channel and distortion effects. By ascertaining an indication of the interference and noise on available communication resources, the most reliable communication resource may be selected for the communication. This indication is known as signal usability.
In geographic reuse communication systems, signal usability is typically limited by the quantity of co-channel interference present on the RF channel. Co-channel interference occurs when receivers receive unwanted information signals from neighboring communication units, or base stations, transmitting on the same channel as the desired RF channel. For a detailed discussion of a method for determining signal usability based on a ratio of the desired signal (C) to the summed quantity of co-channel interference (I) and noise (N) refer to a co-pending United States Patent application, entitled "A Method And Apparatus For Determining Signal Usability", assigned to Motorola Inc. having the same filing date as this application and referenced by applicant's docket number CM01662H. While this technology provides many advantages, it does not address the technological concern of dynamic range limitations imposed by transmitter and receiver distortion effects on determined signal usability.
Distortion effects have several causes with one common result, they tend to limit the maximum achievable signal usability indication (C/(I+N)). Distortion effects add to the interference term in the denominator of the carrier to interference plus noise ratio to produce a new indication of signal usability, C/(I+N+D), where D represents the distortion effects produced by both the receiver and the transmitter. Typically, the co-channel interference is much larger than the distortion effects; thus, the distortion effects can be neglected and the indication of signal usability can be obtained by evaluating the carrier to interference plus noise ratio. When the co-channel interference becomes small, the distortion effects impact the signal usability indication and prevent a linear correlation between the actual signal usability and the carrier to interference plus noise ratio. Therefore, without acknowledging the effects of distortion, the range of accurate signal usability indications determined from evaluating the carrier to interference plus noise ratio is limited to a maximum value due to the implications of the distortion effects. In a logarithmic representation, the maximum measurable carrier to interference plus noise ratio is typically less than 25 dB due to inherent distortions of present technology. However, some frequency reuse communication systems require maximum signal usability indications in excess of 30 dB for optimal system operation. In these wide dynamic range systems, estimating distortion effects is critical to obtaining accurate indications of signal usability and enhancing system functionality.
Distortion effects commonly encountered in transmitters and receivers include timing errors in digital receivers, carrier feedthrough, filter distortions, and amplifier nonlinearities. Timing errors result from improper sampling of a received signal and promote intersymbol interference. Carrier feedthrough degrades the desired signal by displacing a portion of the RF carrier energy into the modulated information signal. Filter distortions modify the magnitude and phase of the desired signal due to their inherently variant magnitude and group delay frequency responses. Amplifier nonlinearities, such as intermodulation distortion, introduce unwanted RF energy into the bandwidth of the desired signal.
Therefore, a need exists for a method and apparatus that mitigate the effects of distortion to enhance the dynamic range of signal usability determinations.